Chariklo & Chiron: Centaurs with Possible Rings

byPaul GilsteronMarch 19, 2015

You may be forgiven if you aren’t familiar with the name Chariklo. Discovered in 1997, 10199 Chariklo is a ‘centaur,’ an outer system body with an orbit that moves between the orbits of Saturn and Uranus, just nudging the orbit of the latter. Its odd name (we’re big on names and their derivations here) comes from a nymph who in Greek mythology was the wife of Chiron and daughter of Apollo. No centaur is larger than Chariklo (estimated diameter 250 kilometers), and until just the other day, no other centaur was known to have what Chariklo did: A system of rings.

We’ve just learned, though, that the second largest centaur, 2060 Chiron, may have a set of rings of its own, although there are alternative ways of interpreting the data. Whether Chiron’s rings are confirmed or not, what was once thought to be an unusual phenomenon, a feature of Saturn alone, is now turning out to be far more common, with rings known to orbit Jupiter, Uranus and Neptune as well as Chariklo. So we have full-scale planets with rings of various size and density, and centaurs, which have been found to share the characteristics of both comets and asteroids. There may well be more ring systems out there, as there are estimated to be 44,000 centaurs in the Solar System with a diameter larger than 1 kilometer.

Image: An artist’s impression of the dense and narrow rings around Chariklo. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris. Data from a recent occultation implies that another centaur, Chiron, may also have a ring system, although other explanations are still being examined. Credit: European Southern Observatory.

The discovery of rings around Chariklo and the work on Chiron came about because of stellar occultations as the objects in question passed in front of a bright star, Chariklo in 2013 and Chiron in 2011. In both cases, the occultation produced a useful light signature for researchers studying the brief shadow. A disk of debris circling Chiron is one reading of the data from that centaur’s occultation, but we can’t rule out jets of material from the surface or even a shell of gas and dust enveloping the object. Amanda Bosh (MIT), a co-author of the paper on the discovery in the journal Icarus, calls that an intriguing result because of Chiron’s location, “…part of that middle section of the solar system, between Jupiter and Pluto, where we originally weren’t thinking things would be active, but it’s turning out things are quite active,”

Traces of activity on Chiron actually trace back to the early 1990s, when MIT’s James Elliot studied a similar stellar occultation by the centaur and was able to make estimates of its size. At that time, features in the data also implied the existence of jets of water and dust coming from the centaur’s surface. The new data, drawn from NASA’s Infrared Telescope Facility on Mauna Kea and the Las Cumbres Observatory Global Telescope Network at Haleakala, give us a more precise readings on an event that in its entirety lasted no more than a few minutes.

MIT’s Jessica Ruprecht, lead author of the paper, notes the range of possibilities involved in this work:

“If we want to make a strong case for rings around Chiron, we’ll need observations by multiple observers, distributed over a few hundred kilometers, so that we can map the ring geometry. But that alone doesn’t tell us if the rings are a temporary feature of Chiron, or a more permanent one. There’s a lot of work that needs to be done.”

According to this MIT news release, the two features observed in the data are each about 300 kilometers from the center of the object and are not dissimilar to what Elliot observed in the 1990s. These may be symmetrical jets of gas and dust rather than rings, perhaps the result of the centaur’s having moved inward from the Kuiper Belt, warming enough to turn frozen gases into jets that throw dust and other material off the surface. Ruprecht also notes that debris from a nearby object could conceivably be captured by a centaur like Chiron to produce rings. At the moment, then, we don’t know if we’re looking at a long-lasting feature or a transient event.

Back to Chariklo, whose stellar occultation in 2013 revealed the existence of two rings, one about 3 kilometers and the other about 7 kilometers wide, separated by about 9 kilometers. The find was startling because no previous ring systems around minor bodies had been discovered, and there were questions about how stable a ring system could be around such a small object. The two rings have received nicknames (Olapoque for the larger, Chui for the smaller) derived from the names of rivers in Brazil, but the IAU will at some point confer official names on both.

Could these rings be formed py plume material reaching orbital speed? Speaking of plumes, Ceres appears (MAYBE) to have one. According to the LATEST NASA briefing, spectral analysis strongly favors ICE at the bright spots. The spots appear to brighten during the day, and then TURN OFF COMPLETELY at night; just what you would expect from a slowly rotating COMET!

Subliming ice from an upwelling of water/brine from cracks below the crater? I believe if there is life on Ceres we will most likely find it there in that ice dome and it is easy to get too. Finding life on Ceres maybe even answer a question whether panbiotica occurs and its resulting implications for the rest of the Cosmos.

We know that Rosetta has detected a population of material that is bound into a >100km orbit around 67P so maybe a similar mechanism is at work at Chiron to produce a spherical shell? This, or rings, may fit the occultation curve better than supposing there are two similar but directly opposite plumes needed to explain the symetrical dimming. How perfect would these plumes need to align to give similar readings over the two dates mentioned for Chiron observations? A shell or ring system would be more stable over a longer period than the variability of venting, surely?

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last eleven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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